The present invention relates to production of fresh water from seawater or other brackish water by desalination.
Desalination is the process of converting seawater, which contains 35,000 parts per million of salt, or brackish water, containing 4,000 to 10,000 parts per million of salt, to fresh water suitable for human consumption, household and industrial requirements. The salt in drinking water should not exceed 500 parts per million. Regardless of the source, water containing dissolved solids and other undesired substances may be referred to as contaminated water.
Production of fresh water by many types of evaporators and reverse osmosis devices has been known. However the equipment and facilities required by these processes have been very expensive and troublesome when used continuously on a large scale. For example, as temperatures exceed 160 degrees Fahrenheit in an evaporator, fouling of the water separation surfaces by scale deposits from the seawater or brackish water occur as insoluble compounds, such as calcium and magnesium salts, are left behind as water is removed. These deposits interfere with the operation and the thermodynamic efficiency of the desalination plant. In addition, operation and maintenance of such prior desalination facilities is also costly and time consuming, as these facilities require technicians to monitor complicated multi-step processes and clean and repair a large amount of process unit equipment. Improvement in efficiency and desalination system design to lower energy needs and eliminate costly evaporator fouling and maintenance requirements is needed.
The present invention provides a novel desalination apparatus and method which, after the system reaches operating temperatures and pressures, requires only relatively small amounts of additional heat energy to sustain the desalination process. In one embodiment of the present invention, a desalination plant includes a heat exchanger in which incoming seawater receives heat from fresh water leaving the desalination plant, a pressure vessel which receives the heated seawater, a water distribution unit comprising a distribution platform and a plurality of silicone strips hanging below the platform. The heated seawater is distributed to the tops of the silicone strips, and flows by gravity down the surface of the silicone strips. As the seawater flows down the silicone strips, it spreads across the strips, increasing its surface area to promote evaporation of water from the seawater. In addition, the seawater flowing down the silicone strips is also heated by the high temperature steam in the atmosphere within the pressure vessel, enhancing evaporation of the water from the seawater on the silicone strips. The water released from the seawater enters the pressure vessel atmosphere, where it can be drawn off for recovery as fresh water.
As the now-concentrated salts in the brine produced from evaporation of the water reaches the ends of the silicone strips, it falls to the bottom of the pressure vessel. From this location, the salts may be removed by conventional techniques, such as pumping or use of a salt auger. The high purity sea salts remaining after removal of the remaining water from the brine may be marketed and sold, providing a valuable income stream which helps increase the economic efficiency of the desalination plant. Alternatively in the case of seawater, the sea salts may be returned to the body of water from which they originated. As an alternative to use of a device such as a salt auger, the bottom of the pressure vessel may be equipped with at least a pair of sliding blade sets which permit controlled salt extraction. One of the blade sets sits on the bottom of the pressure vessel, and in their normal position block off a plurality of holes through the bottom of the pressure vessel. The other blade set is disposed on the outside of the pressure vessel, and are also arranged to permit the blades to block off the plurality of holes. Through coordinated movement of the blade sets, salt may be extracted from the bottom of the pressure vessel on an as-desired basis. Due to the desire to maintain positive control over the flow of salt from the pressure vessel to prevent blow-out of steam and water from the steam environment above the salt pile, it would be preferred to have the movement of the blade sets be coordinated by a computer controller, with the blade sets' movement controlled to occur only during predefined plant conditions.
An additional benefit of this embodiment of the present invention is that because the chemicals and minerals introduced with the seawater do not adhere to the silicone strips, there is no significant build-up of fouling on the strips. This naturally self-cleaning system provides a significant reduction in personnel and maintenance costs, and helps to provide long operating cycles between system shutdowns.
As energy is transferred from the high temperature steam within the pressure vessel environment to the seawater being introduced into the pressure vessel, an energy source must replenish the heat energy in the steam environment to maintain the desalination process. Preferably, the supplemental energy is provided by a heat addition device which does not require high maintenance, such as a steam generator. In other embodiments, the supplemental heat energy source may include other energy sources which, when preferably coupled with a steam generator, provide an energy efficient approach to supplying sufficient volumes of high temperature and pressure steam.
In a further embodiment, as a portion of the steam in the primary evaporation chamber pressure vessel is drawn off to be condensed into fresh water and delivered from the desalination plant, the steam may pass from the pressure vessel to a condensation chamber within a secondary pressure vessel via a turbine. The turbine extracts energy from the steam, both to condition the steam prior to condensation, and to provide a source of power (electrical and/or mechanical) which may be used to operate the desalination plant, further enhancing overall plant efficiency, and/or provided to outside consumers.
In an alternative embodiment, in place of the above-mentioned distribution tank and silicone chords, the incoming seawater or brackish water may be dispensed from a feed inlet onto the upper surface of a shape, such as a cone, coated with silicone or TEFLON® to resist contaminant adhesion. As with the above-noted silicone chords, as the seawater is dispersed across the surface of the cone it receives heat energy from the surrounding steam environment to evaporate at least a portion of the water from the seawater. Advantageously, in addition to the heat energy received from the steam environment within the evaporation space, additional heat energy may be added to the seawater by heating the underside surface of the cone from below with steam injected into the space underneath the cone. Further heat transfer to the seawater may be obtained by extending the surface on which the seawater is spread, downward from the outer periphery of the cone shape in a generally cylindrical fashion. This vertical cylindrical surface, also coated with an adhesion-resistant material, not only provides additional surface area over which heat energy may be added to the seawater from the steam environment, it also helps contain the steam injected in the region beneath the cone shape against the interior surfaces of the seawater heating assembly for a longer period, enhancing heat transfer to the surfaces and the seawater.
The novel desalination apparatus and operating method of the present invention offers significant advantages over previous desalination units, including reducing desalination plant operating costs, both in terms of reduced maintenance and staffing needs, and high energy efficiency as after initial start-up, the present invention requires only supplemental energy inputs into the system in order to maintain the seawater evaporation process on the silicone strips. A further advantage of the present invention is that hard-to-remove chemical compound deposits are virtually eliminated on the desalination process equipment, because contaminates such as calcium and magnesium salts do not readily adhere to silicone and thus readily fall off of the silicone strips to the bottom of the pressure vessel.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.